This invention relates to ferroresonant voltage regulating circuits, and particularly to circuits of such type which have an adjustable output voltage or closed feedback loops.
Ferroresonant regulators presently find widespread use in the power supply field. Among the many advantages of such type regulator is the fact that it is a regulating transformer circuit which provides voltage isolation and allows setting of the output voltage level. In addition, such type of regulator is reliable, of relatively low cost, simple in structure and of small size, provides excellent voltage regulation with static and dynamic input line voltage changes, has inherent short circuit protection, has good efficiency and input power factors, has output characteristics which protect rectifiers, requires a smaller filter than for other types of sources, and has multiple output capability. Summarily the ferroresonant regulator power supply is the simplest, lowest cost, and most reliable power supply available today for producing large amounts of regulated DC or AC power from an AC source.
Ferroresonant voltage regulators basically include a linear inductor, a saturating inductor, more commonly called a saturating reactor, and a capacitor. The linear inductor is in series with the input line to the voltage regulator and the saturating reactor shunts the output. The capacitor, often called a ferroresonating capacitor, or more simply a ferrocapacitor, shunts the saturating reactor and is usually tuned near resonance with the linear inductor. Both the linear inductor and the saturating reactor may be wound on a single transformer core with the input and output electrically isolated, in which case, the input winding is on a non-saturating portion of the transformer core and the output winding is on a saturating portion. In each half cycle of AC input the saturating core saturates, and the impedance of the saturating winding drops. The capacitor resonates with the low, saturated inductance to quickly discharge the saturating winding and recharge in the opposite polarity. The core thereupon drops out of saturation so that further ringing does not occur. The AC output, which may be rectified to provide DC output, is taken from across the ferrocapacitor. When the ferrocapacitor voltage reverses, therefore, the output voltage reverses and the output half cycle is terminated. A saturating core, however, requires a fixed volt-time area of its saturating winding characteristic in order to saturate. Consequently, when the input voltage increases or decreases, the core saturates earlier or later in the immediate half cycle, but the volt-time product of each half cycle of output voltage is constant. When the input frequency is constant, therefore, providing a constant steady state volt-time average per output half cycle, the output voltage must be constant. As a result, changes in input voltage have little effect on output voltage and regulation against changes in input voltage is obtained thereby.
For a given design, it is well known that the output voltage is directly proportional to the source frequency since the volt-time area across the core is held constant. It is also well known that the output voltage changes with temperature because the saturation flux density is temperature dependent. Also, the impedances in the load winding and load circuit cause changes in the output voltage when the load current is changed. Manufacturing tolerances in the transformer circuit and tolerances in the saturation flux density of the magnetic core material cause changes in the output voltage for a fixed design. There is no convenient way of adjusting the output voltage or controlling the output voltage to correct for variables once the design has been made.
Many methods have been applied in an attempt to provide some degree of adjust or control in the ferroresonant regulator field. Some prior methods are mentioned in my U.S. Pat. No. 3,739,257 issued June 12, 1973 and assigned to North Electric Company, Galion, Oh. That patent also discloses a variable flux-reset ferroresonant voltage regulator having simplified adjustment or control type capabilities, and which further has improved output regulating characteristics. The structure comprises a single transformer or magnetic component having two separate saturating core portions both decoupled from the source by separate magnetic shunt means, with the ferroresonant capacitor coupled across windings on both of the saturating portions. The circuit is thyristor controlled with a separate thyristor across the winding of each saturating core portion, one thyristor being enabled by a control circuit during the first half cycle to connect the capacitor in series with the other saturating core portion to resonately discharge and reverse charge the capacitor, and the other thyristor is selectively enabled during the other half cycle. The patent discloses a thyristor controlled ferroresonant voltage regulator circuit in which the output voltage is made adjustable by varying the reset level of each of two parallel magnetic core paths upon which the load windings are wound. One magnetic core path is driven hard into magnetic saturation during one half cycle of the output waveform, and the second magnetic core path is clamped at a given value; in the second half cycle the second path is driven into magnetic saturation and the one path is clamped at the given value. The level of clamping is determined by an associated control circuit which may comprise a simple manually adjustable potential source, or a circuit with load sensing and automatic feedback capabilities.
The ferrocapacitor is discharged at the end of each half cycle of the output voltage and recharged in the opposite polarity. The characteristics of the resonant discharge and recharge is determined by the shape of the B-H characteristic. For high quality grain-oriented silicon steel, the magnetic saturation region is very flat requiring a large magnetizing force to change the flux a small amount. This results in a very low impedance across the windings on the two saturating core portions when the core enters the saturation region. The resultant discharge current in the ferroresonant capacitor has a very high peak value and short duration.